1. Field of the Invention
This invention relates in general to phosphorescent screens and in particular to phosphorescent screens which are useful for such purposes as intensifying screens for radiographs. More specifically, this invention relates to phosphorescent screens which comprise a layer of finely-divided phosphor particles dispersed in a polymeric binder, to a process for production of such screens in which heat-curing of the binder is utilized, and to a coating composition for use in the manufacture of the aforesaid screens.
2. Description of the Prior Art
It is well known to prepare phosphorescent screens by coating a support with a composition comprising phosphor particles, a polymeric binder and a solvent, and drying the coating to remove the solvent and leave an adherent layer comprised of phosphor particles dispersed in the binder. A wide variety of polymeric materials have been disclosed in the prior art for use as a binding agent for the phosphor particles. Among such materials are the polyvinyl butyral of U.S. Pat. No. 3,043,710, the nylon resins of U.S. Pat. No. 3,300,310, the acrylic acid/alkyl acrylate copolymers of U.S. Pat. No. 3,300,311, the polycarbonates of U.S. Pat. Nos. 3,617,285 and 3,712,827, and the polyurethane elastomers of U.S. Pat. No. 3,743,833. Typically, the process of forming the screen involves the use of an organic solvent to form a dispersion of phosphor in binder and employs a drying step in which the solvent is removed by evaporation either at room temperature or at an elevated temperature. However, aqueous systems can also be used as in the case of the water-soluble copolymers of U.S. Pat. No. 3,300,311 and in U.S. Pat. No. 3,776,754 which describes a process for manufacture of screens for color television tubes in which corpuscular radiation is employed to cure selected areas of a radiation-curable layer formed by coating an aqueous composition containing binder and phosphor.
Prior methods of forming phosphorescent screens which comprise a layer of phosphor particles dispersed in a polymeric binder suffer from serious disadvantages which significantly restrict the usefulness of the resulting materials. For example, the phosphor layer may not adhere well to the support, or while having adequate adhesive strength, it may lack the necessary cohesive strength. Due to the brittle character of some binders, the screen may not possess adequate flexibility and resistance to cracking and crazing. The radiographic speed of the screen may be unduly low due to the adverse effects of the binder, or the binder may not be able to accept sufficiently high loadings of phosphor particles. Because of inadequate durability of the phosphor layer or insufficient resistance to soiling, it may be essential to have a protective overcoat layer and because of the poor flatness characteristics of the screen, it may be essential to have an anti-curl layer. Such additional layers add substantially to the cost of the product and greatly complicate the manufacturing process. Still other problems associated with prior art phosphorescent screens are inadequate dimensional stability, discoloration upon aging, and excessive change in properties with changes in temperature and/or humidity. Moreover, many of the prior art phosphorescent screens require manufacturing processes which are unduly slow or difficult to carry out, and some require the use of toxic and hazardous solvents which must be evaporated in substantial quantities.
Improved phosphorescent screens having a highly desirable combination of properties not possessed by screens known theretofore are disclosed in Lu et al, U.S. Pat. No. 4,188,449, issued Feb. 12, 1980. These screens are manufactured by a process comprising the steps of (1) coating a support with a radiation-curable composition comprising a suspension of finely-divided phosphor particles in a viscous liquid composition containing a first component that is capable of being radiation-cured to form a cross-linked polymeric matrix surrounding the phosphor particles and a second component that is capable of being evaporated to generate voids within the matrix, (2) irradiating the coating to cure the first component and form thereby a cross-linked polymeric matrix surrounding the phosphor particles, and (3) evaporating the second component simultaneously with or subsequently to the irradiating step to thereby generate voids within the matrix. While screens prepared by this process represent an important advance in the art, they suffer from certain significant disadvantages which have limited their usefulness. For example, the radiation-cured phosphor layer is not soluble in common solvents, which makes it very difficult to recover phosphor from coating scrap and thereby adds significantly to manufacturing costs.
It is toward the objective of providing phosphorescent screens which possess the advantageous combination of properties of the screens of U.S. Pat. No. 4,188,449, while additionally exhibiting solubility in common solvents which facilitates recovery of phosphor from coating scrap, that the present invention is directed.